To test the hypothesis that 13C-retention from 13C-lysine tracer increases, the lower the dietary lysne intake, and that it reaches a plateau at a specific dietary lysine intake below which no further homeostatic response occurs (i.e. maximal retention). Our objectives are: 1) To develop a "lysine 13C-retention score" which would be used to assess the nutritional status for lysine in humans. 2) To use the data for developing a metabolic model of lysine metabolism and homeostasis.